Electrostatic actuators are preferred over other types of actuators in the applications where power consumption and weight are critical, Also, electrostatic actuation is the most suitable for being used in large arrays of actuators. Electrostatic actuators require low power, operate at high speed, and can be fabricated in a small size, presenting significant advantages over thermal, electromagnetic and piezoelectric actuators.
Electromagnetic actuation requires heavy magnetic materials and relatively large currents, and the construction of such devices is not compact enough to be suitable for actuation of large surfaces.
Electrothermally induced actuation is structurally suited for activator arrays, but has the drawbacks of high power requirements, low speed of response and, in many cases dependence on environmental temperatures.
Piezoelectric actuation, while structurally fitted for actuator arrays and uses low power with adequate or high speed, does not possess the displacements needed and are, in fact, so low as to be not useful for the above considered applications. Piezoelectric materials with increased performance have been proposed, but are both very expensive and heavier than would be acceptable.
In electrostatic actuators, the desired displacement is the result of the attractive electrostatic force generated by the interaction between a distribution of opposite sign charges placed on two bodies, one of which is moveable. For the purposes of this invention, these two bodies are known as actuator plates. The actuator plates are placed apart by a predetermined distance. The charge distribution is then generated by applying a potential difference between two conductive electrodes that are part of the actuator plates. The actuator will be in the ON state or mode when a potential difference is applied between the electrodes and will be in the OFF state when the electrodes are at the same potential.
In some electrostatic actuators, the actuator plates have to come in intimate contact during the normal operation cycle. These actuators are sometimes referred to as touch-mode electrostatic actuators. In order to prevent electrical shorting during the touch phase of the operation cycle, the conductive electrodes are isolated from each other by dielectric layers. In order to get maximum work from a specific device, large electric fields are usually developed between the two conductive electrodes. The non-linear character of the electrostatic attraction results in a snapping action, where the actuator plates move toward each other with accelerations as high as 10.sup.8 g and speeds that exceed 10.sup.3 m/sec. After the impact, the free surfaces of the actuator plates are pushed against each other by the large electrostatically generated pressure.
In electrostatic actuators such as valves, switches, relays, mirrors and shutters, the actuated electrodes have to stay in contact during the ON state. For long ON cycles, interface interactions, charge injection and trapping introduce difficulties of operation. The strong interaction forces being developed between the actuator plates can continue to act after removal of the potential difference between the actuator plates. In some cases, these forces are stronger than the restoring forces available for bringing the electrodes in their original position. In such a case, the two electrodes remain temporarily or permanently attached and the actuator stops functioning as intended and desired. This condition is sometimes referred to as `stiction.` Long contact may even cause permanent stiction, rendering the device completely inoperative.
Using special driving voltages and hydrophobic coatings, it has been possible to increase the life time of touch-mode electrostatic actuators by several orders of magnitude. However, even now, when the actuators are kept in the ON state, or with contact between actuator plates, for long periods of time, such as a few days or longer, stiction still occurs. It would be of great advantage to the art if a device could be provided which overcame the problem of stiction.
It would be another great advance in the art if stiction could be avoided in systems without disturbing the system's requirement to remain in the ON mode for very long periods of time.
Other advantages will appear hereinafter.